Using carbon dioxide
from the atmosphere, as well as sunlight and water, hybrid poplar trees
grow fast and tall, up to 12 feet per year. They also harbor a considerable
amount of carbon in their stems, branches, leaves, and roots. Plant geneticists
would like to design hybrid poplar trees that maximize the amount of carbon
they store in their cell walls. These trees could then be used to more
effectively sequester carbon dioxide, a greenhouse gas, through increased
carbon storage in their roots and, after the roots decay, in soil. Alternatively,
when harvested and digested microbially, these "designer" trees could
offer an increased yield of commodity chemicals (e.g., polylactic acid,
furfural, and acetic acid) and ethanol fuel.

To
produce a hybrid poplar tree, flowers from the female, Populus
trichocharpa, are isolated and inoculated with pollen from the
male, Populus deltoides. Hybrid progeny grow from the resulting
seeds. The flowers and pollen from the hybrid trees can be crossed
to produce descendants of the "grandparent" trees. The genomes of
the grandparent trees and their progeny can be compared to a deck
of cards that is shuffled and reshuffled for each tree. In the grandparents,
the male has all black cards and the female has all red cards. In
the next generation, the cards are half red and half black. In the
third generation, the percentages of black and red cards vary greatly
for each tree, producing genetic diversity that allows the linkage
of genes to highly desirable traits.

In trees, carbon
is "allocated" between aboveground stems, branches, and leaves and belowground
roots. It is "partitioned," or divided, among three types of plant cell-wall
componentscellulose, hemicellulose, and lignin. A plant could be
designed to have an unusually high cellulose content above ground, if
increased ethanol production is desired. In addition, if carbon sequestration
is the goal, its roots could be designed to have unusually high lignin
content, which is resistant to degradation by microbes, increasing the
residence time of carbon in the soil.

"In five years, we hope
to determine which genes control carbon allocation and partitioning in
hybrid poplar trees," says Gerald Tuskan, a plant geneticist in ORNL's
Environmental Sciences Division (ESD). "Our research indicates that carbon
allocation is controlled by a small number of regulatory genes, that separate
genes controlling cell-wall chemistry operate independently above ground
and below ground, and that genes controlling carbon allocation affect
carbon partitioning."

Tuskan is working
on a three-year project to enhance bioenergy conversion and carbon sequestration
in woody plants with his ESD colleagues Stan Wullschleger, Tim Tschaplinski,
and Lee Gunter; Brian Davison of the Chemical Technology Division; and
several researchers from DOE's National Renewable Energy Laboratory. The
team is studying wood tissue samples from some 300 hybrid poplars grown
in Washington that are the progeny of trees from Minnesota and Oregon
parents.

Tuskan and his colleagues
are mapping the hybrid poplar genome by finding genetic "markers" unique
to trees that have a desirable trait, such as higher-than-normal cellulose
content above ground. A marker is a known DNA sequence associated with
a particular gene or trait; in this study, it consists of two unique,
non-repeating DNA sequences flanking simple sequence repeats, such as
GAGAGAGAGA. Some 150 markers have been found so far; the project's goal
is 400 markers.

"Each hybrid poplar
tree has a unique genetic fingerprint," Tuskan says. "We look for an association
between markers unique to each tree and variations in the allocation and
partitioning of carbon content. Once we find the marker that controls
the trait we are interested in, such as high lignin content in the roots,
then we will try to locate the genes responsible. Such genes could be
used to design tree root systems that are high in lignin content."

Tuskan is also interested
in finding the genes that control the size and thickness of a tree's cell
walls, the substructure of wood that determines its usefulness and commercial
value. "It's because of differences in cell sizes and wall thicknesses
that oak floors are stronger than pine floors, maple furniture is more
attractive than aspen furniture, and white oak rather than red oak is
used to make barrels to store wine," he says. "Cell dimensions also determine
whether a tree's wood is suitable for combustion or production of paper
or ethanol."

Use of a light microscope
or scanning electron microscope to determine wood cell dimensions in samples
from various trees is expensive and time consuming. So, Tuskan sought
help from Mike Paulus of ORNL's Instrumentation and Controls Division.
Paulus is a co-developer of the high-resolution, X-ray-computed tomography
system called a MicroCAT scanner. Although used mostly to image internal
defects in small animals, the MicroCAT scanner also offers a faster, better,
and cheaper way to measure the lengths and diameters of cell walls in
wood. (See MicroCAT "Sees" Hidden
Mouse Defects.)

"With the MicroCAT,
we can get cell measurements from an intact block of wood, whereas for
microscope studies, we have to slice wood into very small pieces," Tuskan
says. "With the light microscope, we were getting 100-micron resolution,
but with the modified MicroCAT, we get 10-micron resolution and may be
able to get down to a resolution of one to two microns. The MicroCAT is
a great tool for rapidly screening for wood-cell dimensions in the context
of a large genetic mapping study."